1. Field of the Invention
The subject invention relates to the operation of gas turbine engines, and more particularly, to a control system for a turboshaft engine of a helicopter which includes a wide-band speed governor.
2. Background of the Related Art
In rotary wing aircraft, and more particularly, in helicopters, the main and tail rotors define the primary flight control surfaces for the aircraft. The rotor drive train for the main and tail rotor is coupled to the power plant which can have a single or twin engine configuration. Engine response, is therefore critical to the control of the aircraft. Even more critical to the control of the aircraft is fast fuel flow response to the engine. Thus, it is desirable to provide a rotor speed control system with a high bandwidth enabled by increased proportional and derivative power turbine governing gains. This can be achieved by appropriate filtering of the speed feedback signal.
It is also desirable to provide a rotor speed control system that has increased main and tail rotor resonant frequency attenuation as compared to prior art control systems. It would also be desirable to provide a rotor speed control system that eliminates the need for yaw and lateral cyclic load anticpation.
These and other desirable attributes are achieved as part of the subject invention by providing a high-order filter in the speed control loop of the engine control system disclosed herein.
The subject invention is directed to a new and useful control system for a turboshaft engine of a helicopter. The control system includes a speed control loop with means for generating a power turbine speed signal based upon a demanded rotor speed, means for filtering the power turbine speed signal by effectuating a rapid attenuation of main and tail rotor torsional frequencies in the power turbine speed signal without compromising phase at low frequencies, and a governor for providing isochronous power turbine speed and rotor speed control based upon the filtered power turbine speed signal.
In accordance with the subject invention, the means for filtering the power turbine speed signal is a high-order filter. Preferably, the high-order filter is an eighth order filter. In an embodiment of the invention, the high-order filter is configured as three second order filters cascaded in series with two first order filters. Alternatively, the high-order filter is configured as a sixth order filter in series with a second order filter.
The speed control system of the subject invention further includes an optional active torsional damping loop for damping main and tail rotor torsional frequencies. The torsional damping loop includes a Kalman state estimator for estimating a plurality of engine states based upon one or more measured engine states, such as, for example power turbine shaft torque (QS). The torsional damping loop also includes a Linear Quadratic Regulator (LQR) that provides combustive damping.
These and other aspects of the subject invention will become more readily apparent to those having ordinary skill in the art from the following detailed description of the invention taken in conjunction with the drawings described hereinbelow.